The present invention relates to a process for the catalytic gas-phase oxidation of propene to acrolein, in which a reaction gas starting mixture comprising propene, molecular oxygen and at least one inert gas, at least 20% by volume of which consists of molecular nitrogen, and containing the molecular oxygen and the propene in a molar ratio O2:C3H6 of xe2x89xa7l is passed over a fixed-bed catalyst, whose active material is at least one molybdenum- and/or tungsten- and bismuth-, tellurium-, antimony-, tin- and/or copper-containing multimetal oxide, in such a way that the propene conversion in a single pass is xe2x89xa790 mol % and the associated selectivity of the acrolein formation and of the acrylic acid byproduct formation together is xe2x89xa790 mol %.
The abovementioned process for the catalytic gas-phase oxidation of propene to acrolein is generally known (cf. for example EP-A 15565, EP-A 700714, DE-C 2830765, DE-C 3338380, JP-A 91/294239, EP-A 807 465, WO 98/24746, EP-B 279374, DE-C 2513405, DE-A 3300044, EP-A 575897 and DE-A 19855913) and is important in particular as the first oxidation stage in the preparation of acrylic acid by two-stage catalytic gas-phase oxidation of propene in two reaction stages in series (cf. for example DE A 3002829). Acrylic acid is an important monomer which is used as such or in the form of its alkyl esters for the production of polymers suitable, for example, as adhesives.
Since a small amount of acrylic acid byproduct is usually formed in the abovementioned catalytic gas-phase oxidation of propene to acrolein and, according to the above, acrylic acid is as a rule the desired natural secondary product of acrolein, the molar sum of acrolein formed and acrylic acid formed as byproduct is usually considered as desired product in a catalytic gas-phase oxidation of propene to acrolein. This approach is also to be applicable in the present patent application.
The object of any catalytic fixed-bed gas-phase oxidation of propene to acrolein is in principle to obtain a very high space-time yield (STY) of desired product (in a continuous procedure, this is the amount of desired product produced in grams per hour and unit volume of the catalyst bed used in liters).
There is therefore general interest in carrying out the gas-phase oxidation with a very high loading of the catalyst bed with propene (this is understood as meaning the amount of propene in liters under standard temperature and pressure conditions (=l(S.T.P.); the volume in liters which the corresponding amount of propene would assume under standard temperature and pressure conditions, i.e. at 25xc2x0 C. and 1 bar) which is passed as a component of the reaction gas mixture, per hour, through one liter of catalyst bed), without significantly impairing the propene conversion taking place in a single pass of the reaction gas starting mixture through the catalyst bed and the selectivity of the associated formation of desired product.
The implementation of the abovementioned is impaired by the fact that gas-phase oxidation of propene to acrolein on the one hand is highly exothermic and on the other hand is accompanied by a multiplicity of possible parallel and subsequent reactions.
With increasing loading of the catalyst bed with propene, and with realization of the desired boundary condition of an essentially constant propene conversion, it must therefore be assumed that the selectivity of the formation of desired product decreases as a result of the greater heat production (cf. EP-B 450596, Example 1 and 2).
The conventional processes for the catalytic gas-phase oxidation of propene to acrolein, wherein nitrogen is used as a main component of the inert diluent gas and in addition a fixed-fed catalyst present in a reaction zone and homogeneous along this reaction zone, i.e. having a chemically uniform composition over the catalyst bed, is employed and the temperature of the reaction zone is kept at a value standard over the reaction zone (temperature of the reaction zone here is understood as meaning the temperature of the catalyst bed present in the reaction zone when the process is carried out in the absence of a chemical reaction; if this temperature in such reaction zone is not constant, the term temperature of the reaction zone means here the numerical average of the temperature of the catalyst bed along the reaction zone), therefore limit the applicable value of the propene loading of the catalyst bed to xe2x89xa6155 l(S.T.P.) of propene per l of catalyst bed per h (cf. for example EP-A 15565 (maximum propene load=120 l(S.T.P.) of propene/lxc2x7h), DE-C 2830765 (maximum propene load=94.5 l(S.T.P.) of propene/lxc2x7h), EP-A 804465 (maximum propene load=128 l(S.T.P.) of propene/lxc2x7h), EP-B 279374 (maximum propene load=112 l(S.T.P.) of propene/lxc2x7h), DE-C 2513405 (maximum propene load=110 l(S.T.P.) of propene/lxc2x7h), DE-A 3300044 (maximum propene load=112 l(S.T.P.) of propene/lxc2x7h), EP-A 575897 (maximum propene load=120 l(S.T.P.) of propene/lxc2x7h), DE-C 3338380 (in essentially all examples, the maximum propene load is 126 l(S.T.P.) of propene/lxc2x7h; only in the case of a special catalyst composition was a propene load of 162 l(S.T.P.)/lxc2x7h realized) and DE-A 19855913 (maximum propene load=155 l(S.T.P.) of propene/lxc2x7h)).
WO 98/24746 considers it necessary, even at a propene loading of up to 148.8 l(S.T.P.) of propene/lxc2x7h, to structure the catalyst bed in such a way that its volume-specific activity increases gradually in the direction of flow of the reaction gas mixture.
Although JP-A 91/294239 discloses, in an exemplary embodiment, that a propene load of the catalyst bed with 160 l(S.T.P.) of propene/lxc2x7h is possible in an essentially conventional procedure for a catalytic gas-phase oxidation of propene to acrolein, this is likewise only at the expense of a volume-specific activity gradually increasing in the direction of flow of the reaction gas mixture. However, such a procedure is not very practicable on an industrial scale, the gas-phase catalytic oxidation of propene to acrolein usually being carried in tube-bundle reactors comprising a few thousand catalyst tubes, each individual one of which has to be loaded with a gradated catalyst bed.
EP-B 253409 and the associated equivalent, EP-B 257565, disclose that, with the use of an inert diluent gas which has a higher molar heat capacity than molecular nitrogen, the propene content of the reaction gas starting mixture can be increased. Nevertheless, in the two abovementioned publications, the maximum realized propene loading of the catalyst bed is 140 l(S.T.P.) of propene/lxc2x7h.
Only in EP-A 293224 have propene loadings above 160 l(S.T.P.) of propene/lxc2x7h been realized to date. However, this has been achieved at the expense of a special inert diluent gas to be used, which is completely free of molecular nitrogen. The disadvantage of this diluent gas is in particular the fact that, in contrast to molecular nitrogen, all its components are desired products which, when the process is carried out continuously, have to be recycled at least partially to the gas-phase oxidation in an expensive manner for cost-efficiency reasons.
EP-B 450596 used a structured bed of catalyst and obtained a propene loading of 202.5 l(S.T.P)/lxc2x7h, but at the cost of reduced selectivity to desired product.
It is an object of the present invention to provide a process, as defined at the outset, for the catalytic gas-phase oxidation of propene to acrolein, which process ensures a higher space-time yield with respect to desired product without having the disadvantages of the high-load procedures of the prior art.
We have found that this object is achieved by a process for the catalytic gas-phase oxidation of propene to acrolein, in which a reaction gas starting mixture comprising propene, molecular oxygen and at least one inert gas, at least 20% by volume of which consists of molecular nitrogen, and containing the molecular oxygen and the propene in a molar ratio O2:C3H6 of xe2x89xa71 is passed, at elevated temperatures, over a fixed-bed catalyst, whose active material is at least one molybdenum- and/or tungsten- and bismuth-, tellurium-, antimony-, tin- and/or copper- (preferably at least one Mo-, Bi- and Fe-)containing multimetal oxide, in such a way that the propene conversion in a single pass is xe2x89xa790 mol % and the associated selectivity of the acrolein formation and of the acrylic acid byproduct formation together is xe2x89xa790 mol %, wherein
a) the loading of the fixed-bed catalyst with the propene contained in the reaction gas starting mixture is xe2x89xa7160 l(S.T.P.) of propene per l of catalyst bed per h,
b) the fixed-bed catalyst consists of a catalyst bed arranged in two spatially successive reaction zones A, B, the temperature of the reaction zone A being from 300 to 390xc2x0 C. (frequently to 350xc2x0 C.) and the temperature of the reaction zone B being from 305 to 4200xc2x0 C. (frequently up to 380xc2x0 C.) and at the same time being at least 5xc2x0 C. above the temperature of the reaction zone A,
c) the reaction gas starting mixture flows first through the reaction zone A and then through the reaction zone B and
d) the reaction zone A extends to a propene conversion of from 40 to 80 mol %.
Preferably, the reaction zone A extends to a propene conversion of from 50 to 70, particularly preferably from 65 to 75, mol %.
According to the invention, the temperature of the reaction zone B is advantageously from 305 to 365xc2x0 C., or 340xc2x0 C., particularly advantageously from 310 to 3300xc2x0 C.
Furthermore, the temperature of the reaction zone B is preferably at least 10xc2x0 C. above the temperature of the reaction zone A.
The higher the chosen propene loading of the catalyst bed in the novel process, the greater should be the chosen difference between the temperature of the reaction zone A and the temperature of the reaction zone B. Usually, however, the abovementioned temperature difference in the novel process will be not more than 50xc2x0 C., i.e. the difference between the temperature of the reaction zone A and the temperature of the reaction zone B may be, according to the invention, up to 20xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 40xc2x0 C., up to 45xc2x0 C. or up to 50xc2x0 C.
As a rule, the propene conversion, based on the one pass, in the novel process is xe2x89xa792 mol % or xe2x89xa794 mol %. The selectivity of the formation of desired product is usually xe2x89xa792 mol % or xe2x89xa794 mol %, frequently xe2x89xa795 mol % or xe2x89xa796 mol % or xe2x89xa797 mol %.
Surprisingly, the abovementioned applies not only in the case of propene loadings of the catalyst bed of xe2x89xa7165 l(S.T.P.)/lxc2x7h or of xe2x89xa7170 l(S.T.P.)/lxc2x7h or xe2x89xa7175 l(S.T.P.)/lxc2x7h or xe2x89xa7180 l(S.T.P.)/lxc2x7h, but also in the case of propene loadings of the catalyst bed of  greater than 20 185 l(S.T.P.)/lxc2x7h or xe2x89xa7190 l(S.T.P.)/lxc2x7h or xe2x89xa7200 l(S.T.P.)/lxc2x7h or xe2x89xa7210 l(S.T.P.)/lxc2x7h and in the case of loadings of xe2x89xa7220 l(S.T.P.)/lxc2x7h or xe2x89xa7230 l(S.T.P.)/lxc2x7h or xe2x89xa7240 l(S.T.P.)/lxc2x7h or  greater than 250 l(S.T.P.)/lxc2x7h.
In this respect, it is surprising that the abovementioned values are achievable even when the inert gas used according to the invention comprises xe2x89xa730% by volume or xe2x89xa740% by volume or xe2x89xa750% by volume or xe2x89xa760% by volume or xe2x89xa770% by volume or xe2x89xa780% by volume or xe2x89xa790% by volume or xe2x89xa795% by volume of molecular nitrogen. In the case of propene loadings above 250 l(S.T.P.)/lxc2x7h, the presence of inert (inert diluent gases are intended in general to be those which undergo less than 5%, preferably less than 2%, conversion during a single pass) diluent gases such as propane, ethane, methane, pentane, butane, CO2, CO, steam and/or noble gases, is recommended for the novel process. However, these gases and their mixtures can of course also be used in the case of lower loadings and as sole diluent gases. It is furthermore surprising that the novel process can be carried out using a catalyst bed which is homogeneous, i.e. chemically uniform, over both reaction zones, without suffering significant declines in the conversion and/or in selectivity.
In the novel process, the propene loading usually will not exceed 600 l(S.T.P.)/lxc2x7h. In the novel process, without significant loss of conversion and selectivity, the propene loadings are typically xe2x89xa630, frequently xe2x89xa625, l(S.T.P.)/lxc2x7h.
In the novel process, the operating pressure may be either below atmospheric pressure (for example up to 0.5 bar) or above atmospheric pressure. Typically, the operating pressure is from 1 to 5 bar, frequently from 1.5 to 3.5, bar. Usually, the reaction pressure will not exceed 100 bar.
According to the invention, the molar O2:C3H6 ratio in the reaction gas starting mixture must be xe2x89xa71. Usually, this ratio is xe2x89xa63. According to the invention, the molar O2:C3H6 ratio in the reaction gas starting mixture is frequently xe2x89xa71.5 and xe2x89xa62.0.
A suitable source of the molecular oxygen required in the novel process is air as well as air depleted in molecular nitrogen (for example xe2x89xa790% by volume Of O2 and xe2x89xa610% by volume of N2).
According to the invention, the propene content of the reaction gas starting mixture may be, for example, from 4 to 15, frequently from 5 to 12, % by volume or from 5 to 8% by volume (based in each case on the total volume).
Frequently, the novel process is carried out at a volume ratio of propene to oxygen to inert gases (including steam) in the reaction gas starting mixture of 1:(1.0 to 3.0):(5 to 25), preferably 1:(1.7 to 2.3):(10 to 15).
Usually, the reaction gas starting mixture contains essentially no further components apart from said constituents.
Suitable fixed-bed catalysts for the novel process are all those whose active material is at least one Mo-, Bi- and Fe-containing multimetal oxide.
This means in principle that all those catalysts which are disclosed in DE-C 3338380, DE-A 19902562, EP-A 15565, DE-C 2830765, EP-A 807465, EP-A 279374, DE-A 3300044, EP-A 575897, U.S. Pat. No. 4438217, DE-A 19855913, WO 98/24746, DE-A 19746210 (those of the formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714 can be used according to the invention. This applies in particular to the exemplary embodiments in these publications, among which those of EP-A 15565, of EP-A 575897, of DE-A 19746210 and of DE-A 19855913 are particularly preferred. A catalyst according to Example 1c of EP-A 15565 and a catalyst which is to be prepared in a corresponding manner but whose active material has the composition Mo12Ni6.5Zn2Fe2Bi1P0.0065K0.06Ox.10SiO2 are particularly noteworthy in this context. The example with the consecutive No. 3 from DE-A 19855913 (stoichiometry: Mo12Co7 Fe3 Bi0.6K0.08Si1.6Ox) as an unsupported catalyst in the form of hollow cylinders and measuring 5 mmxc3x973 mmxc3x972 mm (external diameterxc3x97heightxc3x97internal diameter) and the unsupported multimetal oxide II catalyst according to Example 1 of DE-A 19746210 are also noteworthy. The multimetal oxide catalysts of U.S. Pat. No. 4438217 should also be mentioned. The latter applies in particular when these hollow cylinders measure 5 mmxc3x972 mmxc3x972 mm or 5 mmxc3x973 mmxc3x972 mm or 6 mmxc3x973mmxc3x973mm or 7 mmxc3x973mmxc3x974 mm (in each case external diameterxc3x97heightxc3x97internal diameter).
A multiplicity of the multimetal oxide active materials suitable according to the invention can be subsumed under the formula I
Mo12BiaFebXc1Xd2Xe3Xf4Onxe2x80x83xe2x80x83(I), 
where
X1 is nickel and/or cobalt,
X2 is thallium, an alkali metal and/or an alkaline earth metal,
X3 is zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and/or tungsten,
X4 is silicon, aluminum, titanium and/or zirconium,
a is from 0.5 to 5,
b is from 0.01 to 5, preferably from 2 to 4,
c is from 0 to 10, preferably from 3 to 10,
d is from 0 to 2, preferably from 0.02 to 2,
e is from 0 to 8, preferably from 0 to 5,
f is from 0 to 10 and
n is a number which is determined by the valency and frequency of the elements in I other than oxygen.
They are obtainable in a manner known per se (cf. for example DE-A 4023239) and are usually molded as such into spheres, rings or cylinders or used in the form of coated catalysts, i.e. premolded inert carriers coated with the active material. However, they can of course also be used in powder form as catalysts. According to the invention, it is of course also possible to use the Bi-, Mo- and Fe-comprising multimetal oxide catalyst ACS-4 from Nippon Schokubai.
In principle, active materials suitable according to the invention, in particular those of the formula I, can be prepared in a simple manner by producing, from suitable sources of their elemental constituents, a very intimate, preferably finely divided dry blend having a composition corresponding to their stoichiometry and calcining said dry blend at from 350 to 650xc2x0 C. The calcination can be carried out either under inert gas or under an oxidizing atmosphere, e.g. air (mixture of inert gas and oxygen) or under a reducing atmosphere (for example, a mixture of inert gas, NH3, CO and/or H2). The duration of calcination may be from a few minutes to a few hours and usually decreases with increasing temperature. Suitable sources of the elemental constituents of multimetal oxide active materials I are those compounds which are already oxides and/or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.
In addition to the oxides, suitable starting compounds of this type are in particular halides, nitrates, formates, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and/or hydroxides (compounds such as NH4OH, (NH4)2CO3, NH4NO3, NH4CHO2, CH3COOH, NH4CH3CO2 and/or ammonium oxalate, which decompose and/or can be decomposed, at the latest during the subsequent calcination, into compounds which escape completely in gaseous form, may additionally be incorporated into the intimate dry blend).
The thorough mixing of the starting compounds for the preparation of multimetal oxide materials I can be effected in dry or in wet form. If it is carried out in dry form, the starting compounds are expediently used in the form of finely divided powders and, after mixing and any compaction, are subjected to calcination. However, the thorough mixing is preferably effected in wet form. Usually, the starting compounds are mixed with one another in the form of an aqueous solution and/or suspension. Particularly intimate dry blends are obtained in the mixing method described when exclusively sources of the elemental constituents in solution are used as starting materials. A preferably used solvent is water. The aqueous material obtained is then dried, the drying process preferably being carried out by spray-drying the aqueous mixture with outlet temperatures of from 100 to 150xc2x0 C.
The multimetal oxide materials suitable according to the invention, in particular those of the formula I, can be used for the novel process both in powder form and after molding to give specific catalyst geometries, it being possible to carry out the shaping before or after the final calcination. For example, unsupported catalysts can be prepared from the powder form of the active material or its uncalcined and/or partially calcined precursor material by compaction to give the desired catalyst geometry (for example by pelleting or extrusion), it being possible to add assistants, such as graphite or stearic acid as lubricants and/or molding assistants and reinforcing agents, such as microfibers of glass, asbestos, silicon carbide or potassium titanate. Suitable unsupported catalyst geometries are, for example, solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of hollow cylinders, a wall thickness of from 1 to 3 mm is expedient. The unsupported catalyst may of course also have spherical geometry, it being possible for the sphere diameter to be from 2 to 10 mm.
The shaping of the pulverulent active material or of its pulverulent, still uncalcined and/or partially calcined precursor material can of course also be effected by application to premolded inert catalyst carriers. The coating of the carriers for the preparation of the coated catalysts is carried out, as a rule, in a suitable rotatable container, as disclosed, for example, in DE-A 2909671, EP-A 293859 or EP-A 714700. For coating the carriers, the powder material to be applied is expediently moistened and, after application, is dried again, for example by means of hot air. The coat thickness of the powder material applied to the carrier is expediently chosen to be from 10 to 1000 xcexcm, preferably from 50 to 500 xcexcm, particularly preferably from 150 to 250 xcexcm.
Conventional porous or nonporous aluminas, silica, thorium dioxide, zirconium dioxide, silicon carbide or silicates, such as magnesium silicate or aluminum silicate, can be used as carrier materials. The carriers may have a regular or irregular shape, those having a regular shape and substantial surface roughness, for example spheres or hollow cylinders, being preferred. The use of essentially nonporous, spherical steatite carriers which have a rough surface and whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, the use of cylinders whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm as carriers is also suitable. In the case of rings suitable according to the invention as carriers, the wall thickness is moreover usually from 1 to 4 mm. Annular carriers preferably to be used according to the invention have a length of from 3 to 6 mm, an external diameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Rings measuring 7 mmxc3x973mmxc3x974 mm (external diameterxc3x97lengthxc3x97internal diameter) are also particularly suitable according to the invention as carriers. The fineness of the catalytically active oxide materials to be applied to the surface of the carrier is of course adapted to the desired coat thickness (cf. EP-A 714 700).
Advantageous multimetal oxide active materials to be used according to the invention are furthermore materials of the formula II
[Y1a,y2b,Ox,]p[Y3c,Y4d,Y5e,Y6f,Y7g,Y2h,Oy,]qxe2x80x83xe2x80x83(II),
where
Y1 is bismuth, tellurium, antimony, tin and/or copper,
Y2 is molybdenum and/or tungsten,
Y3 is an alkali metal, thallium and/or samarium,
Y4 is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and/or mercury,
Y5 is iron, chromium, cerium and/or vanadium,
Y6 is phosphorus, arsenic, boron and/or antimony, and
Y7 is a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and/or uranium,
axe2x80x2 is from 0.01 to 8,
bxe2x80x2 is from 0.1 to 30,
cxe2x80x2 is from 0 to 4,
dxe2x80x2 is from 0 to 20,
exe2x80x2 is from 0 to 20,
fxe2x80x2 is from 0 to 6,
gxe2x80x2 is from 0 to 15,
hxe2x80x2 is from 8 to 16,
xxe2x80x2,yxe2x80x2 are numbers which are determined by the valency and frequency of the elements in II other than oxygen and
p,q are numbers whose ratio p/q is from 0.1 to 10,
containing three-dimensional regions of the chemical composition Y1a,Y2b,)x, which, owing to their composition differing from their local environment, are delimited with respect to their local environment and whose maximum diameter (longest distance between two points present on the surface (interface) of the region and passing through the center of gravity of the region) is from 1 nm to 100 xcexcm, frequently from 10 nm to 500 nm or from 1 xcexcm to 50 or 25 xcexcm.
Particularly advantageous novel multimetal oxide materials II are those in which Y1 is bismuth.
Preferred among these in turn are those which are of the formula III
xe2x80x83[Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3]pxe2x80x3 [Z212Z3cxe2x80x3Z4dxe2x80x3Feexe2x80x3Z5fxe2x80x3Z6gxe2x80x3Z7hxe2x80x3Oyxe2x80x3]qxe2x80x3xe2x80x83xe2x80x83(III)
where
Z2 is molybdenum and/or tungsten,
Z3 is nickel and/or cobalt,
Z4 is thallium, an alkali metal and/or an alkaline earth metal,
Z5 is phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,
Z6 is silicon, aluminum, titanium and/or zirconium,
Z7 is copper, silver and/or gold,
axe2x80x3 is from 0.1 to 1,
bxe2x80x3 is from 0.2 to 2,
cxe2x80x3 is from 3 to 10,
dxe2x80x3 is from 0.02 to 2,
exe2x80x3 is from 0.01 to 5, preferably from 0.1 to 3,
fxe2x80x3 is from 0 to 5,
gxe2x80x3 is from 0 to 10,
hxe2x80x3 is from 0 to 1,
xxe2x80x3,yxe2x80x3 are numbers which are determined by the valency and frequency of the elements in III other than oxygen and
pxe2x80x3,qxe2x80x3 are numbers whose ratio pxe2x80x3/qxe2x80x3 is from 0.1 to 5, preferably from 0.5 to 2,
very particularly preferred materials III being those in which Z2bxe2x80x3 is (tungsten)bxe2x80x3 and Z212 is (molybdenum)12.
It is furthermore advantageous if at least 25 mol % (preferably at least 50, particularly preferably at least 100, mol %) of the total amount [Y1a,Y2b,Ox,]p ([Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3]pxe2x80x3) of the multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention are present in the multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention in the form of three-dimensional regions of the chemical composition Y1a,Y2b,Ox, [Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3] which, owing to their chemical composition differing from their local environment, are delimited with respect to their local environment and whose maximum diameter is from 1 nm to 100 xcexcm.
Regarding the shaping, the statements made with respect to the catalysts comprising multimetal oxide materials I are applicable to catalysts comprising multimetal oxide materials II.
The preparation of active materials from multimetal oxide materials II is described, for example, in EP-A 575897 and in DE-A 19855913.
In a manner expedient in terms of application technology, the novel process is carried out in a two-zone tube-bundle reactor. A preferred variant of a two-zone two-bundle reactor which can be used according to the invention is disclosed in DE-C 2830765. However, the two-zone tube-bundle reactors disclosed in DE-C 2513405, U.S. Pat. No. 3147084, DE-A 2201528 and DE-A 2903218 are also suitable for carrying out the novel process.
This means that, in the simplest procedure, the fixed-bed catalyst to be used according to the invention is present in the metal tubes of a tube-bundle reactor, and two heating media, as a rule salt melts, essentially spatially separated from one another, are passed around the metal tubes. The tube section over which the respective salt bath extends represents, according to the invention, a reaction zone, i.e. in the simplest procedure a salt bath A flows around that section of the tubes (the reaction zone A) in which the oxidative reaction of the propene (in a single pass) takes place until a conversion of from 40 to 80 mol % is reached, and a salt bath B flows around that section of the tubes (the reaction zone B) in which the subsequent oxidative reaction of the propene (in a single pass) takes place until a conversion of at least 90 mol % is reached (if required, further reaction zones which are kept at individual temperatures may follow the reaction zones A, B to be used according to the invention.
It is expedient in terms of application technology if the novel process comprises no further reaction zone, i.e. the salt bath B expediently flows around that section of the tubes in which the subsequent oxidative reaction of the propene (in a single pass) takes place up to a conversion of xe2x89xa792 mol % or xe2x89xa794 mol % or more.
Usually, the beginning of the reaction zone B is behind the hot-spot maximum of the reaction zone A. The hot-spot maximum of the reaction zone B is usually below the hot-spot maximum temperature of the reaction zone A.
According to the invention, the two salt baths A, B can be passed cocurrent or countercurrent through the space surrounding the reaction tubes, relative to the direction of flow of the reaction gas mixture flowing through the reaction tubes. According to the invention, it is of course also possible to employ cocurrent flow in the reaction zone A and countercurrent flow in the reaction zone B (or vice versa).
In all abovementioned configurations, it is of course also possible to superpose a transverse flow on the flow of the salt melt parallel to the reaction tubes, within the respective reaction zone, so that the individual reaction zone corresponds to a tube-bundle reactor as described in EP-A 700714 or in EP-A 700893 and a meandering flow of the heat exchange medium results through the catalyst tube bundle in the longitudinal section as a whole.
Expediently, the reaction gas starting mixture is preheated to the reaction temperature before being fed to the catalyst bed.
In the abovementioned tube-bundle reactors, the catalyst tubes are usually produced from ferritic steel and typically have a wall thickness of from 1 to 3 mm. Their internal diameter is as a rule from 20 to 30 mm, frequently from 22 to 26 mm. It is expedient in terms of application technology if the number of catalyst tubes housed in the tube-bundle container is at least 5000, preferably at least 10,000. Frequently, the number of catalyst tubes housed in the reaction container is from 15,000 to 30,000. Tube-bundle reactors having more than 40,000 catalyst tubes tend to be the exception. Inside the container, the catalyst tubes are usually homogeneously distributed, the distribution expediently being chosen so that the distance between the central inner axes of adjacent catalyst tubes (the catalyst tube spacing) is from 35 to 45 mm (cf. for example EP-B 468290).
Particularly suitable heat exchange media are fluid heating media. The use of melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and/or sodium nitrate or of metals having a low melting point, such as sodium, mercury or alloys of various metals, is particularly advantageous.
In all abovementioned configurations of the flow in the two-zone tube-bundle reactors, the flow rate inside the two required circulations of heat exchange medium is as a rule chosen so that the temperature of the heat exchange medium increases by from 0 to 15xc2x0 C. from the point of entry into the reaction zone to the point of exit from the reaction zone, i.e. the abovementioned xcex94T may be, according to the invention, from 1 to 10xc2x0 C., or from 2 to 8xc2x0 C. or from 3 to 6xc2x0 C.
The temperature of the heat exchange medium on entry into the reaction zone A is, according to the invention, usually from 300 to 350xc2x0 C. According to the invention, the temperature of the heat exchange medium on entry into the reaction zone B is usually, on the one hand, from 305 to 380xc2x0 C. and, on the other hand, is simultaneously at least 5xc2x0 C. above the temperature of the heat exchange medium entering the reaction zone A.
The temperature of the heat exchange medium on entry into the reaction zone B is preferably at least 10xc2x0 C. above the temperature of the heat exchange medium entering the reaction zone A. The difference between the temperatures on entry into the reaction zones A and B may thus be, according to the invention, up to 20xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 40xc2x0 C., up to 45xc2x0 C. or up to 50xc2x0 C. Usually, however, the abovementioned temperature difference is not more than 50xc2x0 C. The higher the chosen propene loading of the catalyst bed during the novel process, the greater should be the difference between the temperature of the heat exchange medium on entry into the reaction zone A and the temperature of the heat exchange medium on entry into the reaction zone B.
Advantageously, the temperature of the heat exchange medium on entry into the reaction zone B is, according to the invention, from 305 to 365xc2x0 C. or 340xc2x0 C., particularly advantageously from 310 to 330xc2x0 C.
In the novel process, the two reaction zones A, B can of course also be realized in tube-bundle reactors spatialy separated from one another. If required, a heat exchanger may also be mounted between the two reaction zones. The two reaction zones A, B can of course also be designed as a fluidized bed.
In the novel process, it is also possible to use catalyst beds whose volume-specific activity in the direction of flow of the reaction mixture increases continuously, abruptly or stepwise (this can be effected as described in WO 98/24746 or in JP-A 91/294239 or by dilution with inert material). Moreover, the inert diluent gases (for example only propane or only methane etc.) recommended in EP-A 293224 and in EP-B 257565 can also be used for the two-zone procedure described. The latter can, if required, also be combined with a volume-specific activity of the catalyst bed which increases in the direction of flow of the reaction gas mixture.
It should once again be pointed out here that, for carrying out the novel process, it is also possible to use in particular the two-zone tube-bundle reactor type which is described in German published application DAS 2,201,528 and includes the possibility of transferring a portion of the hotter heat exchange medium of the reaction zone B to the reaction zone A in order, if required, to heat up a cold reaction gas starting mixture or a cold recycle gas.
The novel process is particularly suitable for being carried out continuously. It is surprising that it permits the higher space-time yield in the formation of the desired product in a single pass without at the same time significantly impairing the selectivity of the formation of the desired product. Rather, a nonsignificantly higher selectivity in the formation of the desired product is even observed.
The latter is presumably due to the fact that, owing to the higher temperatures present in the region of higher propene conversion, the novel process results in less readsorption of the resulting acrolein onto the fixed-bed catalyst.
It is also noteworthy that the catalyst life in the novel process is completely satisfactory in spite of the extreme loading of the catalyst with the reactants.
The novel process does not give pure acrolein but a mixture from whose secondary components the acrolein can be separated off in a manner known per se. Unconverted propene and inert diluent gas used and/or formed in the course of the reaction can be recycled to the gas-phase oxidation. When acrolein is used for the preparation of acrylic acid by two-stage catalytic gas-phase oxidation of propene, the acrolein-containing reaction gases are transferred to the second oxidation stage as a rule without removal of the secondary components. If required, the novel two-zone procedure can of course also be used in the case of conventional propene loads.
Otherwise, conversion, selectivity and residence time are defined as follows in this publication, unless stated otherwise:             Conversion      ⁢              xe2x80x83            ⁢              C        p            ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      propeno      ⁢              xe2x80x83            ⁢              (        %        )              =                            Number          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propene          ⁢                      xe2x80x83                    ⁢          converted`                          Number          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propene          ⁢                      xe2x80x83                    ⁢          used                    xc3x97      100                                                Selectivity            ⁢                          xe2x80x83                        ⁢                          S              A                        ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            acrolein                                                            formation            ⁢                          xe2x80x83                        ⁢                          (              %              )                                            =                                                                      Number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                propene                                                                                        converted                ⁢                                  xe2x80x83                                ⁢                into                ⁢                                  xe2x80x83                                ⁢                acrolein                                                              Number          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propene          ⁢                      xe2x80x83                    ⁢          converted                    xc3x97      100                                                Selectivity            ⁢                          xe2x80x83                        ⁢                          S              AS                        ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            formation            ⁢                          xe2x80x83                        ⁢            of                                                            acrylic            ⁢                          xe2x80x83                        ⁢            acid            ⁢                          xe2x80x83                        ⁢            byproduct            ⁢                          xe2x80x83                        ⁢                          (              %              )                                            =                                                                      Number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                propene                                                                                        converted                ⁢                                  xe2x80x83                                ⁢                into                ⁢                                  xe2x80x83                                ⁢                acrylic                ⁢                                  xe2x80x83                                ⁢                acid                                                              Number          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propene          ⁢                      xe2x80x83                    ⁢          converted                    xc3x97      100                                                Selectivity            ⁢                          xe2x80x83                        ⁢                          S              DP                        ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            formation            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the                                                            desired            ⁢                          xe2x80x83                        ⁢            product            ⁢                          xe2x80x83                        ⁢                          (              %              )                                            =                                                                      Number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                propene                ⁢                                  xe2x80x83                                ⁢                converted                                                                                        into                ⁢                                  xe2x80x83                                ⁢                acrolein                ⁢                                  xe2x80x83                                ⁢                and                ⁢                                  xe2x80x83                                ⁢                into                ⁢                                  xe2x80x83                                ⁢                acrylic                ⁢                                  xe2x80x83                                ⁢                acid                                                              Number          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propene          ⁢                      xe2x80x83                    ⁢          converted                    xc3x97      100                  Residence      ⁢              xe2x80x83            ⁢      time      ⁢              xe2x80x83            ⁢              (        sec        )              =                            Empty          ⁢                      xe2x80x83                    ⁢          reactor          ⁢                      xe2x80x83                    ⁢          volume          ⁢                      xe2x80x83                    ⁢          filled          ⁢                      xe2x80x83                    ⁢          with          ⁢                      xe2x80x83                    ⁢          catalyst          ⁢                      xe2x80x83                    ⁢                      (            1            )                                    Throughput          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          reaction          ⁢                      xe2x80x83                    ⁢          gas          ⁢                      xe2x80x83                    ⁢          starting          ⁢                      xe2x80x83                    ⁢          mixture          ⁢                      xe2x80x83                    ⁢                      (                          1              /              h                        )                              xc3x97      3600      